Enhancement of superconductivity and phase diagram of Ta-doped Kagome superconductor CsV3Sb5

Kagome superconductors AV3Sb5 (A = K, Rb, and Cs) have attracted enormous interest due to the coexistence of charge density wave (CDW) order, unconventional superconductivity (SC) and anomalous Hall effect (AHE). In this paper, we reported an intensive investigation on Cs(V1−xTax)3Sb5 single crystals with systematic Ta doping. Ta was confirmed to be doped into V-site in the Kagome layer from both single crystal X-ray diffraction structural refinement and scanning transmission electron microscopy observation. The highest Ta doping level was found to be about 16%, which is more than twice as much as 7% in Nb-doped CsV3Sb5. With the increase of Ta doping, CDW order was gradually suppressed and finally vanished when the doping level reached to more than 8%. Meanwhile, superconductivity was enhanced with a maximum critical temperature (Tc) of 5.3 K, which is the highest Tc in the bulk crystal of this Kagome system at ambient pressure so far. The μ0Hc2(T) behavior demonstrates that the system is still a two-band superconductor after Ta doping. Based on the electrical transport measurement, a phase diagram was set up to exhibit the evolution of CDW and SC in the Cs(V1−xTax)3Sb5 system. These findings pave a new way to search for new superconductors with higher Tc in the AV3Sb5 family and establish a new platform for tuning and controlling the multiple orders and superconducting states.

The CDW and SC can be tuned by applying external pressure.Wang et al. reported that CDW was suppressed gradually with the increase of pressure and finally vanished, while superconductivity showed nonmonotonic behavior and two SC domes appeared with the maximum Tc enhanced to about 8 K 24 .Similar phenomena were reported independently by several other groups as well [25][26][27][28][29] 4 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing 100081, People's Republic of China. 5Material Science Center, Yangtze Delta Region Academy of Beijing Institute of Technology, Jiaxing 314011, People's Republic of China. 6Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong, People's Republic of China.* email: junli@iphy.ac.cn; hhwen@nju.edu.cn;zhiweiwang@bit.edu.cn or electron is also an effective way to tune CDW and SC.For example, CDW was suppressed very rapidly in Sn-doped CsV 3 Sb 5−x Sn x and almost vanished when the doping level x > 0.05.At the same time, superconductivity exhibits a nonmonotonic evolution with the Sn doping resulting in two "domes" peaked at 3.6 and 4.1 K 30,31 .Yang et al. and Liu et al. reported independently that Ti doping can kill the CDW very quickly, however, the evolution of SC exhibited quite different behavior 32,33 .Tc behaved V-shape in Ref. 32 and single-dome-shape in Ref. 33 as the Ti concentration increased.In addition, Cr and Mo were also reported to be successfully doped into V-site, and both caused the suppression of SC, but CDW showed completely opposite evolution: Cr suppressed CDW while Mo promoted it 34,35 .Our recent work on Nb-doped CVS demonstrated a different situation, with x increased to the solution limit of 7%, Tc was monotonously enhanced up to a maximum of 4.45 K, while CDW was not completely suppressed 36 .Further ARPES study and band calculation revealed an unconventional competition mechanism between SC and CDW in light of the band structure modification near the Fermi surface 37 .
In this letter, we reported the successful growth of Ta-doped Cs(V 1−x Ta x ) 3 Sb 5 single crystals with the highest doping level up to x = 0.16.We investigated evolution of CDW and superconductivity with various doping levels, although samples with the low doping ratio of x ≤ 0.04 have been reported 35 .The Ta doping level was also more than twice as much as in our previous Nb-doped CVS.Both TEM and XRD analysis demonstrated that Ta was effectively substituted for V-site in the Kagome layer.CDW transitions were suppressed gradually with the increase of doping level, and finally vanished when the doping level x is higher than 0.10.Meanwhile, superconductivity was enhanced gradually with the increase of Ta content and the highest Tc of 5.3 K was observed in the x = 0.16 sample, which is the highest value at ambient pressure in the bulk samples of the AV 3 Sb 5 family so far.A comprehensive phase diagram has been established to illustrate the evolution of CDW and SC in the Cs(V 1−x Ta x ) 3 Sb 5 system.

Experiments
Crystal growth Cs(V 1−x Ta x ) 3 Sb 5 (x = 0, 0.04, 0.05, 0.08, 0.1, 0.14 and 0.16) single crystals were synthesized from Cs bulk (Alfa Aesar, 99.8%), V piece (Aladdin, 99.97%), Ta powder (Alfa Aesar, 99.99%) and Sb shot (Alfa Aesar, 99.9999%) via a flux method as described in our previous report.Flux mixtures containing 5 mol percent Cs(V 1−x Ta x ) 3 Sb 5 were loaded into a crucible, and then placed in a quartz tube in an Ar-filled glove box.After the quartz tube was sealed, it was heated slowly to 1000 °C and maintained for 24 h, followed by cooling down to 200 °C at the rate of 3 °C/h.Finally, the furnace was brought down to room temperature with the power switched off.To remove the flux, the obtained samples need to be soaked in deionized water.At last, high-quality single crystals with hexagonal shapes were obtained.We would like to point out that the highest doping level of Ta was x = 0.16, even if we tried to raise the nominal concentration to x = 0.5.

Measurement method
A specimen of Cs(V 0.86 Ta 0.14 ) 3 Sb 5 was used for the single crystal X-ray crystallographic analysis, which was taken from a Bruker D8 QUEST single crystal diffractometer at 293 K, equipped with the APEX III software and Mo radiation.The structure was solved and refined by using the Bruker SHELXTL Software Package to obtain the information on crystal structure.After that, Cs(V 1−x Ta x ) 3 Sb 5 single crystals were structurally and chemically characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu-Kα radiation, and energy dispersive X-ray spectroscopy (EDX) equipped on a JEOL scanning electron microscope (SEM, JSM-7500F) to confirm the content of Ta, respectively.
A physical property measurement system (PPMS, Quantum Design) was used for electronic transport measurements with the temperature from 300 K down to 1.8 K. Cs(V 1−x Ta x ) 3 Sb 5 samples were measured using a five-terminal configuration, in which the longitudinal resistivity (ρ xx ) and the Hall resistivity (ρ yx ) can be measured simultaneously.Temperature dependence of DC magnetic susceptibility was measured in a SQUID magnetometer (Quantum Design MPMS).
Before transmission electron microscope (TEM) analysis, the single crystalline foils with an average thickness of around 50 nm were mechanically exfoliated from the bulk crystal using adhesive tape, and then transferred to the copper grid with the aid of Crystalbond adhesives and acetone.An aberration-corrected JEOL JEM-ARM200CF transmission electron microscope (TEM) was employed to acquire high angle annular dark field (HAADF) image, selected area electron diffraction pattern, and energy dispersive X-ray spectroscopy (EDX) mapping, operated at 200 kV.

Structure characterization
A series of Ta-doped Cs(V 1−x Ta x ) 3 Sb 5 (CVTS) single crystals with x = 0, 0.04, 0.05, 0.08, 0.10, 0.14, and 0.16 were synthesized for the first time.Utilizing the SHELXTL software package for single crystal X-ray diffraction analysis (see section "Method"), the crystal structure of CVTS was obtained, as illustrated in Fig. 1a,b.The CVTS samples maintain the same hexagonal structure as the parent phase, with Ta atoms successfully occupying the V sites as expected.X-ray diffraction pattern on x = 0.14 sample, as shown in Fig. 1c, also confirmed that the crystal structure can be well indexed with hexagonal structure with the space group of P6/mmm, which is the same as CVS single crystal.Comprehensive structural details are summarized in Tables 1, 2. Notably, the lattice parameters a(= b) and c, were calculated to be 5.5587 Å and 9.3020 Å, respectively, show a discernible expansion compared to the parent CsV 3 Sb 5 (a = 5.4949 Å and c = 9.3085 Å).This indicates Ta doping primarily enlarges the ab plane while having a relatively small impact on the c-axis direction.EDS results in Fig. 1d show a clear Ta peak, which means the successful doping of Ta.In addition, subsequent measurements of electronic transport and magnetic properties showed that the superconducting transition temperature and CDW transition temperature changed gradually, also indicating that Ta had been successfully doped.We would like to point out that the doping concentration x adopted in the whole text reflect the actual Ta content determined through EDS analysis, and the doping limit of Ta in CVTS is about 16%, i.e. x = 0.16.
Atomic-resolution STEM and HAADF images of Cs(V 0.86 Ta 0.14 ) 3 Sb 5 which were taken along the [001] axis direction at room temperature, as shown in Fig. 2a, provide further details on the lattice structure.Given the specimen thickness is relatively thin, the brightness of the atomic columns in the HAADF images scales with the specimen thickness and the constituent elements of atomic number Z (approximately ~ Z 1.7 ).Therefore,  www.nature.com/scientificreports/ the relatively heavier elements like Cs and Sb appear brighter than V. Despite Ta being the heaviest among the constituent elements, its low concentration results in negligible contrast alteration.The derived structural model superimposed on the STEM image confirms the isostructure with CsV 3 Sb 5 , and the Kagome lattice can be clearly identified.Both the fast Fourier transform (FFT) (insert in Fig. 2a) and electron diffraction pattern (Fig. 2b) exhibit hexagonal symmetry devoid of any superstructural feature.Besides, to further confirm the successful doping of Ta into the CsV 3 Sb 5 lattice, the EDX elemental mapping was carried and shown in Fig. 2c-g, validates the presence of Ta dopants and homogeneous distribution.

Superconductivity, CDW order and their competition
Electronic transport and magnetic properties measurements were performed to elucidate the interaction between SC and CDW order.Figure 3 shows temperature dependence of resistivity and magnetization of CVTS, one can see a clear competition between SC and CDW with the increasing of Ta content.As the Ta increases, CDW transition temperature progressively decreases from 93.6 K for parent CVS to 40.5 K for sample with x = 0.08, and then faded away completely in samples with the doping content of x = 0.10, 0.14 and 0.16, as shown in Fig. 3a.This behavior is more visible from the dρ/dT curves, as shown in Fig. 3b, which suggests that Ta doping can weaken CDW effectively and ultimately suppress it.Meanwhile, the resistivity at normal state increased with the increase of doping level, this is because the scattering effect could become stronger after Ta doping.Concurrently, superconducting transition temperature Tc was significantly increased, as indicated in Fig. 3c, here Tc is defined as the temperature corresponding to the midpoint of the resistivity drop.With the increasing of Ta, Tc increased gradually and reached to 5.3 K for x = 0.16 sample, which is the highest Tc observed in the bulk of AV 3 Sb 5 family at ambient pressure so far.Since the highest Tc was observed in the sample with the highest Ta content, and neither peak nor saturation was observed as Ta doping, we can expect that Tc could be higher if one can introduce more Ta into CVS by any other methods.To further check superconductivity of CVTS, temperature dependence of magnetization measurement was conducted under an applied magnetic field of 5 Oe from 2 to 10 K.Both the zero-field-cooled (ZFC) and field-cooled (FC) results are shown in Fig. 3d.One can see distinct diamagnetic transitions for all measured samples, which affirmed the occurrence of SC in CVTS.We would like to point out www.nature.com/scientificreports/ that the diamagnetic data here cannot reflect the superconducting volume fraction accurately since we focused on the evolution of Tc and the applied magnetic field is perpendicular to the ab-plane.The higher Ta samples have higher Tc, consistent with that in resistivity measurement.The sharp transitions in both ρ(T) and M(T) curves indicate the high quality of our CVTS crystals.In addition, in the low temperature region(7-50 K) for x = 0.14, the ρ(T) curve can be well fitted using the formula ρ(T) = ρ 0 + AT 2 as shown in the inset of Fig. 3a, where ρ 0 is the residual resistivity, AT 2 term originates from the electron-electron.The fitting gives ρ 0 = 25.4 μΩ cm, A = 3.1 × 10 −3 μΩ cm K −2 .The electron-electron scattering process dominates low-temperature resistivity.The quadratic relationship indicates the normal Fermi liquid behavior 37 .
To examine the evolution of SC under applied magnetic field, we measured the temperature dependence of resistivity at varying magnetic fields on x = 0.14 sample with the fields perpendicular to the ab plane.Figure 4a presents the ρ(T) curves measured from 2 to 6 K under various magnetic fields up to 4 T. One can clearly see a gradual suppression of Tc with the magnetic field increasing.There is still a SC transition sign around 2 K even the magnetic field was applied as high as 3.5 T. But when the field was applied to 4 T, almost no SC transition was detected.To better analyze the resistive transitions and determine upper critical field μ 0 H c2 , the 90%, 50% and 10% levels of normal-state resistivity ρ N (shown by dashed lines) are taken to mark the transition.The difference between these three criteria gives an idea about the uncertainty in determining μ 0 H c2 .The summary of μ 0 H c2 is plotted in Fig. 4b.The two-band theory 38 fits all the three sets of data well and yields μ 0 H c2 (0) to be 4.6 T, 3.2 T, and 2.4 T, corresponding to 90%, 50%, and 10% criteria, respectively.The detailed fitting process was described in Supplementary file.According to the equation of coherence length ξ = √ � 0 /2πµ 0 H c2 , we can estimate the ξ to be 11.7 nm, 10.1 nm, and 8.5 nm, respectively, where 0 is the magnetic flux quantum.
Figure 5 summarize the phase diagram of CVTS single crystals established from the resistivity measurements and the effect of Ta doping on CDW and SC is presented.It is evident that T CDW decreased monotonically with increasing Ta content, and was completely suppressed when x is more than 0.1.While superconductivity was enhanced with the Tc gradually increased spontaneously.Interestingly, the enhancement of SC has two distinct regions: when x is less than 0.08, i.e.CDW order is coexisted with SC, Tc enhanced slowly; however, Tc increased more quickly after CDW was completely suppressed, i.e. in samples with x more than 0.1.Tc does not get saturated as the Ta content increasing.Since the doping limit is 16% in our study by using flux method to grow CVTS crystals, if one can raise the doping limit by using any other different crystal growth techniques, we can expect to obtain higher Tc sample.This competition behavior between CDW and SC seems similar to that in Nb-doped, Ti-doped and Sn-doped CVS.What's different is that CDW was not completely suppressed in Nb-doped case 36 , while there appeared two distinct SC regions in Ti-doped 32,33 and Sn-doped cases 30,31 .These

Figure 1 .
Figure 1.Structure and chemical composition of Cs(V 1−x Ta x ) 3 Sb 5 crystals.(a) Side view and (b) top view of Cs(V 1−x Ta x ) 3 Sb 5 crystal structure.(c) XRD pattern of Cs(V 0.86 Ta 0.14 ) 3 Sb 5 single crystal with (00l) reflections.(d) EDS results of Cs(V 0.86 Ta 0.14 ) 3 Sb 5 sample, the presence of Ta peak confirms Ta was doped into CVS.

Figure 3 .Figure 4 .
Figure 3. Electrical and magnetic properties of Cs(V 1−x Ta x ) 3 Sb 5 .(a) Temperature dependence of longitudinal resistivity from 2 to 300 K.The inset shows the quadratic temperature dependence from 7 to 50 K.(b) Temperature dependence of dρ/dT from 30 to 100 K to illustrate CDW transitions.(c) ρ(T) curves around SC transition temperature.(d) Temperature dependence of magnetic susceptibility for CVTS measured with the applied field of 5 Oe, both ZFC and FC curves are presented.

Figure 5 .
Figure 5. Phase diagram of the Cs(V 1−x Ta x ) 3 Sb 5 crystals.T CDW decreased gradually and finally vanished when x is more than 0.1, while T c increased significantly with x increasing, shows an obvious competition between CDW and SC.

1
. Besides pressurization, chemical doping with hole OPEN Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE), School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China. 2 National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People's Republic of China. 3Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.